Ballantine 340 User manual

90100141A
Printed in U.S.A.
MODEL
340
RF MILLIVOLTMETER
BALLANTINE LABORATORIES, INC.
P.O. BOX 97
BOONTON, NEW JERSEY 07005 USA
201·335·0900/TWX: 71 0·987-8380 JUNE 1972

WARRANTY
All Ballantine Laboratories, Inc. products are
warranted against defects
in materials and work-
manship. This warranty applies for one year from
the date
of delivery, except for vacuum tubes and
batteries, or,
in the case of certain major compo-
nents listed in the instruction manual, for the
specified period.
We will repair or replace products
which prove to be defective during the warranty
period. No other warranty is expressed
or implied.
We are not liable for consequential damages.
CERTIFICATION
The equipment supplied on your order meets all applicable published Ballantine
specifications
as determined by thorough testing and inspection. Ballantine's calibration
measurements are traceable to the National Bureau
of Standards. All instruments used
in calibrating Ballantine products are standardized by systematic reference to NBS-trace-
able standards as described in the validation procedures shown below.
REFERENCE STANDARDS
DC
10mV-750V 0.01-0.04%
20Hz-50kHz
0.5V-500V
0.03%
20Hz-10M
Hz
0.5V-100V 0.1%
DC-30MHz
0.5V-100V
0.5%
DC-700MHz
10W-0.5V 1%- NBS
10MHz-1000MHz
1V-300V 1%- NBS
_t
WORKING STANDARDS
1kHz TRANSFER VM BALLANTINE. D1094
HF TRANSFER VM BALLANTINE. MODEL 393
MICROPOTENTIOMETERS BALLANTINE 440
RATIO TRANSFORMERS. GERTSCH
CALIBRATION CONSOLE 100Hz-100kHz FOR
0.25%
ACCURACY INSTRUMENTS
LEVEL CHECK-DAILY
ATTENUATOR CHECK- 6 MONTH
~
DISTORTION CHECK- 1 MONTH
FREQUENCY RESPONSE CHECK- 3 MONTH
GROUND CURRENT CHECK- 3 MONTH
•
CALIBRATION CONSOLES 2Hz-20MHz FOR
1%-3% ACCURACY INSTRUMENTS
-
LEVEL CHECK- 1 WEEK
.._
ATTENUATOR CHECK- 6 MONTH
DISTORTION CHECK - 3 MONTH
a
FREQUENCY RESPONSE CHECK- 3 MONTH
GROUND CURRENT CHECK- 3 MONTH
lr
CROSSCHECK CONSOLE 2-20MHz FOR
~
1%-3% ACCURACY INSTRUMENTS
LEVEL CHECK- 1 WEEK
.._
ATTENUATOR CHECK- 6 MONTH
DISTORTION CHECK - 3 MONTH
FREQUENCY RESPONSE CHECK- 3 MONTH
GROUND CURRENT CHECK- 3 MONTH
Ballantine's QualityAssuranceprogram satisfied the requirements of MIL-0-9858.

1.
2.
INSTRUCTION BOOK CONTENTS
FOR
MODEL 340 RF MILLIVOLTMETER
GENERAL INFORMATION
Page
1
1.1 Features ................ .. .. .. .. .. .. .. .. .. . .. .. .. .. .. .. .. .. .. . .. .. . .. . .. .. .. . .. .. . .. .. .. .. .... .. .. .. .. .. .... .. 1
1.2 Some Applications of the Model 340 .. .. .. .. .. .... . .. . .. .. . .. .. .. .. .. . .. .. .. .. .. .. . .. .. .. . .. .. .. .. .. . 1
1.3 Technical Characteristics ....................................................................... 1
OPERATION ..................................................................................................... .. 1
1
1
2.1 Power Connection ...........
2.1.1 Line Voltage Conversion .........................................................................
2.2 Starting Procedure ........... ..
1
3
2.3
2.4
2.2.1
Warmup Period .. ..
Function of Controls
Voltage Measurements ..... ..........................................................................
2.4.1 For True RMS Voltage Measurements Below 30 mV ............................ ..
2.4.2 For True RMS Voltage Measurements Above 30 mV ..............................
3
3
3
3
2.5 Possible Error Sources .. .. .... .. .... .. .. .. .. .. . .. .. .... .. . .. .. .... . .. .... .. .. . .. .. .. .. .. .... .. .. .. .... .. .. .. .... 4
2.S.1 Indicator Remains in Extreme Left Position .. .. .... .. . .. .. .. .. . .. . .. .. . .... .. .. .. ... .. . 4
2.5.2 Indicator Remains in Extreme Right Position.......................................... 4
2.5.3 Indicator Reads High and Pointer Stays on &ale for Next Higher Range 4
2.5.4 Indicator Beats or Flutters
.... .................................................................... 4
2.5.5 Loading Errors ......................................................................................... 4
2.5.6 Voltage Drop in Connecting
Leads .......................................................... 4
2.5.7 Stray Field Pickup .................................................................................... 5
2.5.8 Transmission Line Effects ........................................................................ 5
2.5.9 Ground Current Errors ............................................................................ 5
2.5.10 Response and Waveform Errors .............................................................. 5
2.6 Use of the DB
Scale .. .. .. .. .. .. .. .. .. ... .. .. . .. .. .. .. .... .. ... .. .. ... .. .... .. .. .. .. ...... .. .. .. .... .. .... .. 6
2.7 Effect of
DC Component...................................................................................... 6
2.8 Overload Characteristics ..... 6
2.9 Effect of Power Line Voltage Variations .............................................................. 6
2.10 DC Output ............................ ................................................................................ 6
3. CIRCUIT DESCRIPTION ....... .. ............ .. ... .. ........ .. . .. . .. ............ .. .. .. .. .. .. .. . .. .. . .. .. .. . .. . .. 6
3.1 Probe .................................................................................................................... 7
3.1.1 Model 1340 Probe Attenuator ................................................................ 7
3.1.2 Model 2340 Coaxial Adapter .................................................................. 7
3.1.3 Model 4340 Probe
Tip ............................................................................ 7
3.1.4 Model 5340 Tee Adapter ........................................................................ 7
3.1.5 Model 6340 High Voltage Attenuator ...................................................... 7
3.2 Chopper Input .... .. .. . .. .. .. .. .. . .. .. .. .. .... .. .. .. .. .. .. .. .. .... . .. .. .. .. .. .. .. .. . .... .. .. . .. .. . .. .. .. .. .. . .. . .. .. 7
3.3 .. Input Amplifier .................................................................................................... 8
3.4 Midsection Attenuator .......................................................................................... 8
3.5 Output Amplifier and Signal Rectifier .............. ................................................. 8
3.6 Power Supply ........................................................................................................ 8
4. MAINTENANCE ............... .. .. .. .... .. .... .. ......... .. . .. . .. ..... .. .. .... .. .. . .. .. .. .. .. .. . .... .. .. . .. . .. .. . 8
4.1 .General ........ ..................... .............. .................................................................... 8
4.2 Recommended Maintenance ....
8
. I .

5.
6.
7.
4.3 Necessary Maintenance Equipment .
4.3.1 Stable Signal Source
4.3.2 Sensitive AC Vacuum Tube Voltmeter
4.3.3 Sensitive
DC Vacuum Tube Voltmeter .
4.3.4 Tube Checker .
4.4 Removal of the Case .
4.5 2,000 Hour Check .
4.5.1 Scale Adjustment .
4.5.2 Sensitivity Adjustment
4.5.3 Noise Check .
4.5.4 Range Switching Check
4.5.5 DC Output Check
4.5.6 Stability with Line Voltage .
4.6 4,000 Hour Check
4.6.1 Chopper Supply Adjustment
SERVICING AND TROUBLESHOOTING
5.1 Necessary Equipment .
5.2
5.3
5.4
5.5
5.6
5.7
5 .1.1 Stable Signal Source
5.1.2 Sensitive AC Vacuum Tube Voltmeter .
5.1.3 Sensitive DC Vacuum Tube Voltmeter
5.1.4 Tube Checker .
Simple Service Problems .
5.2.1 Removal of the Instrument Case .
5.2.2 Pilot Light Replacement .
5.2.3 Fuse Replacement
5.2.4 Line Voltage Conversion .
General Instructions
Signal Tracing .
Chopper Replacement
Probe Replacement
Troubleshooting Chart .
SHIPPING INSTRUCTIONS .
REPLACEMENT PARTS LIST
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
ILLUSTRATIONS AND DIAGRAMS
Ballantine Model 340 RF Millivoltmeter with Adapters
Rack Panel Mounted Ballantine Model 340-S/2 RF Millivoltmeter
..
Model 340 Location of Controls .
Model 340 Input Impedance
Model 340 Transmission Line Errors
Model 5
340 VSWR .
Figure 7 Model 340 Simplified Schematic . . ...............
Figure 8 Model 340- Right Hand Side View . ....................
Figure 9 Model 340- Left Hand Side View, Input Shield Removed .
8
8
8
8
8
8
8
8
9
9
9
9
9
10
10
10
10
10
10
10
10
10
10
10
10
10
10
11
11
11
11
13
16
III
III
2
4
5
5
6
14
15
Figure 10 Model 340 Wiring Diagram . .. ...................... .. .. .... .. .... .. ... Rear
TABLES
TABLE I DB Level Conversion 6
- II -
Page

Model 1340
Model 2340
Model 5340
Model 4340
Figure 1. Ballantine Model 340 RF Millivoltmeter with Adapters
Figure 2. Rack Panel Mounted Ballantine Model 340-S/2 RF Millivoltmeter
- III -

1. GENERAL INFORMATION
1.1 Features Frequency Range 0.1 Me to > 1000 Me
The Ballantine Model 340 RF Millivoltmeter is a sensitive,
wideband, rms-responding voltmeter with a voltage range
from 300
p. V to 3 V, frequency range of 100 kc to 700 Me,
logarithmic indicator and a basic accuracy of better than
4%.
It brings to high frequency voltage measurements the same
convenience of logarithmic scales, sensitivity and accuracy
that has always been associated with Ballantine voltmeters.
A feature of the Ballantine Model 340
RF Millivoltmeter is
its rms response to distorted sinewave voltages. This is
particularly important at high frequencies because all exist-
ing calibration standards are based on rms-responding de-
vices. High frequency waveforms are often highly distorted
and the means for measuring this distortion are limited.
Signal voltages are measured
in the Model 340 by a probe
which is connected to the voltmeter
by 3 feet of cable. This
allows positioning the probe exactly at the
point at which
the voltage measurement
is desired, thus minimizing errors
due to transmission effects.
The basic
input to the probe is coaxial - a flexible Ys inch
diameter bellows is the center conductor and the outer con-
ductor is a Ys-24 external thread. For input voltages above
30 mV, a probe attenuator (Model 1340) is first connected
to the probe. This probe attenuator has the same flexible
center conductor and threaded outer conductor
as the probe
itself. Customer-designed adapters can be used to make con-
nections to either the probe or the probe attenuator and
probe when the very shortest transmission paths are re-
quired. For less demanding applications the Model 340 is
supplied with a complete set of
input adapters. A coaxial
adapter (Model
2340) allows the probe to be connected to
female Type N and
BNC connectors. A spring-loaded probe
tip (Model
4340) is supplied for in-circuit measurements,
and a Tee-adapter (Model 5340)
is available to match the
probe to a
50 ohm transmission line.
1.2 Some Applications of the Model 340
RMS voltage measurement in grounded circuits
High frequency transistor measurements
Voltage measurements on coaxial systems
High frequency bridge measurements
1.3 Technical Characteristics
Voltage Range 300 p. V to 3 V in 8 ranges of 10 db each
Response
Accuracy
Allowable Max
Crest Factor
Scales
Probe Input
Impedance
Mean Square
DC Output
Input Power
Accessories,
Supplied
Accessories,
Optional
Vacuum Tubes
and
Semiconductors
Dimensions
(inches)
Weight (pounds)
calibrated to 700 Me
True rms, all voltages
Specified in % of reading
at any
point on the scale
0.1 -100 Me ±4%
100 -700 Me ± 10%
300
p.V - 1 mV
1 mV - 3 mV
3 mV -10 mV
10 mV -30 mV
30 mV -100 mV
0.1 v - 0.3 v
0.3 v - 1.0 v
1.0 v - 3.0 v
200 -60
60 -20
20 -6
6 - 2
200 -60
60 -20
20 -6
6 - 2
Two logarithmic voltage scales,
0.95 to 3.3 and 3.0 to 10.6
One linear decibel scale, 0-10
300
p.V -30 mV
30 mV - 3 V >25 kQ and 4 pF
> 1MQand4pF
0.1 to 1.0 V repeated for each 10 db
ac range. 20 kQ source impedance
115/230 V, 50-420 cps, 40 W
Probe Attenuator, Model 1340
Coaxial Adapter,
Model2340
Probe Tip, Model 4340
Tee Adapter, Model 5340
High Voltage Attenuator, Model 6340
2-12AX7, 2-6AU6A, 1-6X4,
1-0A2, 1-0B2WA, 2-2NS25,
2 -S282G, 4-1N816, 5 -1N1692,
2 Probe diodes (BL Special)
Portable:
13 H, 7Y2 W, 9Y2 D
Rack:
SY2 H, 19 W, 8Y2 D
Portable or Rack, 16
Shipping weight: Portable 22, rack 34
2. OPERATION
2.1 Power Connection
The voltmeter is supplied ready to operate on 105 -125 V,
50 cps -420 cps or
210- 250 V, 50 cps -420 cps power line,
as indicated on the decal mounted adjacent to the power cord.
2.1.1 Line Voltage Conversion from 115 V to 230 V
or vice versa is possible. For conversion from 115 V
operation to 230 V operation, connect the power trans-
former leads as follows:
Disconnect black-red lead from terminal 1 and
black-yellow lead from terminal
3, and connect
- 1 •
both together to terminal 2. Replace fuse Fl 0.4
ampere by 0.2 ampere Slo-Blo fuse. Detailed in-
formation for 115 V /230 V conversion is given
on the schematic at the end of this instruction book.
2.2 Starting Procedure
Insert the three-prong plug into a proper ac power outlet.
Use conversion unit where only two-prong outlets are avail-
able. Turn the function switch from OFF position. The pilot
light should glow.
The instrument is ready for use after a
short warmup period.

ADJUSTMENT
PILOT LIGHT
FUSE
Figure 3. Model 340- Location of Controls
- 2 -

2.2.1 Warmup Period
The instrument is usable after approximately 20 sec-
onds. After 15 minutes the indication will be within
1.0% of the final value.
When the instrument has no~ been in use for many
months or has been stored in high humidity allow a
warmup period of at least one hour.
NO SOURCE OF HEAT SHOULD BE CLOSE TO THE
BACK OF THE INSTRUMENT. PERMIT AIR TO CIRCU-
LATE FREELY AROUND THE VOLTMETER TO AVOID
EXCESSIVE TEMPERATURE RISE IN THE INSTRUMENT.
The voltmeter is calibrated in the vertical position. For con-
venience in reading, a tilting device is provided beneath
the case.
2.3 Function of Controls (see fig 3. for control locations)
Function Switch
OFF Turns instrument off.
STANDBY Turns instrument on with the exception
of the chopper. Use this switch position
between series of measurements to pro-
long life of chopper.
METER Turns chopper on and allows instrument
to be used
as a calibrated voltmeter.
NULL Biases meter on-scale with no signal in-
put. Use to set NULL adjustment and for
sensing voltages below the normal mini-
mum reading of 300 microvolts.
Range Selector
FULL
SCALE Selects voltage range of instrument
in 10
db steps. Voltages from 300 tL V to 30 mV
are measured without probe attenuator.
Voltages from 30 mV to 3 V must be
measured with probe attenuator
in place
as well as with correct position of FULL
SCALE range selector.
Panel Adjustments
NULL
SENS
SCALE
With shielded, open-circuit input on 1m V
range, and function switch at NULL,
rotate NULL adjustment control for min-
imum i-ndication. This adjustment should
be made after a
15 minute warmup each
time the instrument is turned on and be-
fore the start of any critical measurements.
Allows a small adjustment
in voltmeter
sensitivity to correct for changes due to
component aging.
Adjusts a biasing current to the indicator
so that full scale on one range will cor-
respond exactly to bottom scale on the
next higher range.
- 3 -
2.4 Voltage Measurements
2.4.1
For True RMS Voltage Measurements
Below 30 mV
Set
Function Switch
Range Selector
To
METER
30 mV
Connect the unknown voltage to the probe input using
the proper adapter
as required and rotate the range
selector counterclockwise until an indication appears
on the meter scale. Take the reading from the scale
indicated by the range selector. Because of the loga-
rithmic meter scale and Ballantine special design, the
accuracy of the measurement is the same at any
point
on the scale.
After a
15 minute warmup, and whenever a series of
critical measurements are to be made
on either the
1
mV or 3 mV range, check the NULL adjustment.
2.4.1.1 NULL Adjustment serves to "zero" or
cancel out small residual voltages in the
input
system of the voltmeter. Since these residuals
tend to be a function of time and temperature,
the adjustment should be made and checked
frequently for greatest accuracy.
To make the NULL adjustment:
Set To
Function Switch NULL
Range Selector 1 mV
Shield the probe input- for example, by con-
necting the Coaxial Adapter, Model 2340, to the
probe
but not screwing it all the way on to con-
nect the center conductors. Allow a few minutes
for any heat generated in handling the probe to
dissipate. Rotate the NULL adjustment for a
minimum indication.
The minimum indication should fall between
1 - 3 db on the meter scale. A higher null
is
an indication of excessive noise or thermal po-
tentials and should be investigated. (See para-
graph 4.5.3)
2.4.2 For True RMS Voltage Measurements
Above
30 mV
Set
Function Switch
Range Selector
Attach
Probe Attenuator, Model 1340
To
METER
3 v
To
Probe
Connect the unknown voltage to the probe attenuator
input using the proper adapter as required and rotate
the range selector counterclockwise until an indication
appears
on the meter scale. Take the reading from the
scale indicated by the range selector.
After a
15 minute warmup and whenever a series of
critical measurements are to be made
on either the
0.1 V or 0.3 V range, check the NULL adjustment (see
paragraph 2.4.1.1)

2.5 Possible Error Sources
The causes of some measuring difficulties are listed below:
2.5.1 Indicator Remains in Extreme Left Position
This condition may be caused by:
a) Power cord disconnected
b) Fuse open
c) Function switch in OFF or STANDBY
position
d) Input voltage lower than range selected
e) Failure to remove Probe Attenuator for
measurements below 30 mV
2.5.2 Indicator Remains in Extreme Right Position
This condition may be caused by:
a) Input voltage higher than range selected
b) Failure to attach Probe Attenuator for
measurements above 30 mV
c) Excessive external fields
d) Excessive ground currents
2.5.3 Indicator Reads High and Pointer Stays on
Scale for Next Higher Range
This condition may be caused by:
a) Function switch in NULL position
b) Excessive external fields
c) Excessive ground currents
2.5.4 Indicator Beats or Flutters
This condition may be caused by:
a) Large spurious input voltage at or near 94 cps
b) Strong magnetic fields such as exist adjacent to
certain power transformers and line voltage
regulators
c) Noisy or unstable input signal
Because of the wide frequency range and high sensltlvtty
of the Model 340, other measuring errors may be introduced
when proper measuring techniques are not used.
The most
common error sources are listed below. A comprehensive
analysis of errors is given in NBS Conference
on Standards
and Electronic Measurements paper "Techniques and Errors
in High Frequency Voltage Calibration," by Dr. E. Uiga
and
W. F. White. A copy of this paper is available without
charge from Ballantine Laboratories.
2.5.5 Loading Errors can occur whenever a source of
emf with other than zero source impedance is measured
by a voltmeter with other than infinite
input imped-
ance. Current drawn by the voltmeter produces a volt-
age drop in the source impedance and the resulting
measured terminal voltage differs from the emf by the
amount
of this voltage drop.
The Model 340 is a high input impedance device. For
input voltages below 30 m V, the low frequency input
impedance can be represented
as more than 25,000
- 4 -
ohms in parallel with 4 pF. Above 30 mV the shunt
resistance can be considered infinite while the capaci-
tance remains approximately at 4 pF. However, at high
frequencies this situation changes. Both the resistive
and capacitive components
of input impedance are
frequency sensitive -shunt resistance decreases with
frequency, shunt capacitance increases. This is shown
in figure
4.
50k
30
20
IOk
7
3
2
I k
0.7
0.5
0.3
0.2
0.1
' '
--
I I ',
~T RESISTANCE
'
'
IOpF
7
......_
h \
INPUT CAPACITANCE \
5------
--""'r-------'r'"- ------ --- -
_,
3 \
2 I I
"'
'
-INPUT < 30mV '\. I
---INPUT > 30mV
'
0.1 0.5 5 I0
50 100 200 400 700
FREQUENCY IN Me
Fig. 4. Model 340 Input Impedance
For any critical voltage measurement it is necessary to
check for loading errors by determining the actual
source impedance
of the emf at the frequency of inter-
est and, with the aid of figure 4, the input impedance
of the Model 340. The relationship between source
emf, Es, terminal voltage,
Vr, source impedance, Zs,
and voltmeter impedance, Zr, is given by:
Zs
Es = Vr (1 + Zr )
2.5.6 Voltage Drop in Connecting Leads can cause
appreciable errors at high frequencies when the induc-
tive reactance of these leads becomes large and the
input impedance of the voltmeter has decreased from
its low frequency value.
The situation is similar to the
loading error effects discussed in 2.5.5.
In this case
the impedance of the connecting leads
is analogous to
the source impedance.
Coaxial connections
i:o the Model 340 should be made
whenever possible to reduce the error due to voltage
drop in connecting leads. The Model 2340 Coaxial
Adapter, supplied with the Model 340, converts the
probe
input to accept female type N or BNC connect-
ors. It can be used, as can all the adapters, both with
and without the probe attenuator for measurements
over the entire voltage range.
The Model 4340 Probe Tip with its associated ground
lead should not be used at higher frequencies if errors
due to the voltage drop in connecting leads is suspected.
The inductance of the ground lead is approximately
0.1 1-'H which at 100 Me represents a reactance of
63 ohms.

2.5.7 Stray Field Pickup is particularly troublesome
at high frequencies. Any loop in the signal input cir-
cuit can have a voltage induced in it that is proportional
to the area of the loop, the stray field density, and
the frequency.
Again, coaxial connections are the best when stray field
pickup
is suspected. Location and removal of the of-
fending sources should be tried, and every attempt
made to reduce the loop area at the signal input.
2.5.8 Transmission Line Effects have to be consid-
ered whenever the distance between the voltage to be
measured and the Model 340 probe input
is not negli-
gibly short compared to a wavelength. The Model 340
is a high impedance device and will therefore introduce
reflections on any transmission line. These reflections
produce a voltage standing wave pattern. The magni-
tude of the voltage standing wave radio determines the
maximum difference that can exist between the source
of voltage to be measured and that indicated by the
Model 340. The actual difference will depend
on the
distance between the source and the Model 340.
Figure 5 shows the error that will exist when the
Model 340
is connected to the voltage source by a
short section of 50 ohm coaxial transmission line. This
set of curves
is derived from transmission line equations
and the input capacitance of the Model 340 (see figure
4).
%ERROR
§::§. XIOO
Es
30
LENGTH (L) IOcm 5 3 3
_I I I I~'""""
i/
:I , I
OF PROBE OR
PROBE
-[-"LENGTH (L) ATTENUATOR
I I!/
INPUT I
~c/
I '
E
MODEL 340 ij
I
Ill\ PROBE!
I '
If
I
I
1---- ER BELOW 30mV ·'l
,),
!j
·--ER ABOVE 30mV 1
I
20
1
I
I
I
I
I em
If
I I
10
f
~J
/V
,)
//
,
~
~
v v
,~~~ ...
~
...
::, b.:=
-~
...
.,.:::.;
0
20 30 50 70 100 200 300 500 700
FREQUENCY IN Me
ER _ I L•=1.2XI0-sf(cpslL(cm)
~ -COSL0- aS1Nl
0 a
::;;;; wC~o
Fig. 5. Model 340 Transmission Line Errors
Transmission line effects can be minimized by shorten-
ing the length of connection between source and Model
340. This may require the construction of specially
threaded adapters that will connect to the ground of
the Model 340 probe.
Transmission line effects may also be minimized
by
measuring in a matched 50 ohm transmission system.
For this application the Model 340 probe is first con-
nected to the Model 5340 Tee Adapter which is then
connected to the
50 ohm transmission line where the
measurement
is desired. The maximum vswr of this
. 5 •
system as a function of frequency is shown in figure 6.
This tabulation shows the maximum errors that can
exist due to reflections caused by the Model 340, assum-
ing the transmission line itself is perfectly matched.
1.30
1.20
0::
3=
en
>
1.10
1.00
/
L
/ v
f.--
~
v
,/
--
-v
100 200 300
FREQUENCY IN Me
Fig. 6. Model 5340 VSWR
INPUT >30mV
I
I
I
v
I INPUT
<37
/ v
500 700
2.5.9 Ground Current Errors can be particularly
troublesome whenever low-level, high frequency volt-
ages are measured. Special circuit precautions have
been made
in the Model 340 to reduce the effects of
ground currents -either currents of power line fre-
qunecy or high frequencies. However, particularly with
high frequency ground currents, it
is impossible to
completely eliminate these effects. Therefore every ef-
fort should be made to reduce the leakage from signal
sources into the power line ground. Avoid positioning
power line cords close to the Model 340 or to any low-
level sources that are being measured. Work at the
highest possible measurement levels.
2.5.10. Response and Waveform Errors are mini-
mized in the Model 340 because the instrument re-
sponds to the rms value of the input signal. This feature
is particularly important in the calibration of the Model
340. All high frequency calibration devices (Micro-
potentiometers, A-T Voltmeters, HF Transfer Volt-
meters) are rms-responding. When they are used to
calibrate a Model 340, errors due to distortion in the
signal sources will be minimized.
The rms feature of the Model 340
is likewise important
when measuring unknown voltages. Distortion
in the
voltage will not lead to the uncertain reading of a peak-
responding or average-responding instrument. How-
ever, particularly at high frequencies, response and
waveform errors must still be considered when using
the Model 340.
If the signal being measured is con-
nected to the Model 340 over a finite length of trans-
mission line -which in practice is always the case-
and if the signal contains higher frequency harmonics,
these harmonics will be transmitted over a longer elec-
trical length than the fundamental. This longer length
will alter the amplitude of the harmonics at the input
of the Model 340, in relationship to the amplitude of